Strictly test debug NaN printing only on x86_64 (#15910)
Strictly test debug NaN printing only on x86_64 (#15910)

NaNs are mangled in various ways on various platforms.  Some
of the fun possibilities:
* sign bits can be cleared
* payloads get zeroed
* NaNs get forced to quiet
* Merely passing a value into a function can trigger these behaviors
* Trivial changes to the generated assembly can change this

This makes it very hard to validate the printing of NaN
details, since we can't be certain what value the formatter
actually saw.

So now I only do exact checks of the NaN formatter on x86_64,
which has pretty well-understood NaN support.  On all other
platforms, I just check that the debugDescription output
has the expected format:
```
   -?s?nan(\(0x[0-9a-f]+\))?
```



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